Hydrology

Hydrological and substance retention studies

Retention of substances (sediments, nutrients, pollutants) in river flood meadows

In 2002 and 2003, a working group of the Institute for Geographical Sciences at the Free University (FU) Berlin carried out a study on matter retention in river flood meadows. The background for this study was the renaturation of rivers and river flood meadows, something, which has become very topical following the recent severe floods in Germany. During flooding, there is a complex interplay between vegetation, geomorphological structure of the riverbed, currents, sedimentation and erosion. The drainage and flow conditions modified by vegetation determine both the course of a flood as well as the distribution of sediments in the flooded area.

As a supplement to field studies in the Upper Rhine area within the framework of the EU project RipFor (Riparian Forest Management), the workgroup measured the flow effects between wetland meadow grasses in pool segments of an outdoor stream under highly-controlled conditions using three-dimensional flow meters. Contrary to the common view, an increase of flow velocity was measured in the water close to the bottom.

This finding has to be taken into account in renaturation measures: a linear arrangement of grass may increase erosion processes during floods. For technical reasons it is not possible to carry out detailed examinations of this kind in field studies, they are only possible in controllable test facilities such as the FSA.